PP1773_unit_alt_grid is a polygon shapefile of surface altitudes for 16 hydrogeologic units as described in report Professional Paper 1773. Surface altitudes were interpolated and discretized to the 2-mile by 2-mile rotated grid used to simulate groundwater flow in the Atlantic Coastal Plain of North and South Carolina. Altitudes are available for 17 surfaces, hydrogeologic unit layer 1 (top - land surface) through Layer 16 (top and bottom).

Each hydrogeologic unit is referred to as its model layer number as represented in the report PP1773. For clarity, they are listed below along with the aquifer unit or confining unit name in North Carolina and its correlated name in South Carolina L1 Surficial aquifer L2 Yorktown confining unit / Upper Floridan confining unit L3 Yorktown aquifer / Upper Floridan aquifer L4 Castle Hayne - Pungo River confining unit / Middle Floridan confining unit (To be referred to as "Castle Hayne confining unit / Middle Floridan confining unit" in this document) L5 Castle Hayne - Pungo River aquifer / Middle Floridan aquifer (To be referred to as "Castle Hayne aquifer / Middle Floridan aquifer" in this document) L6 Beaufort confining unit / Gordon confining unit L7 Beaufort aquifer / Gordon aquifer L8 Peedee confining unit / Crouch Branch confining unit L9 Peedee aquifer / Crouch Branch aquifer L10 Black Creek confining unit / McQueen Branch confining unit L11 Black Creek aquifer / McQueen Branch aquifer L12 Upper Cape Fear confining unit / Charleston confining unit L13 Upper Cape Fear aquifer / Charleston aquifer L14 Lower Cape Fear confining unit / Gramling confining unit L15 Lower Cape Fear aquifer / Gramling aquifer L16 Lower Cretaceous confining unit and aquifer